The present invention relates to a permanent magnet element intended especially for electrotechnical devices, the element comprising at least one permanent magnet piece. The invention also relates to an electric machine as described in the preamble of claim 11.
Permanent magnets are generally used in electric machines for generating magnetic fields and further for utilizing the interaction between a magnet field and conductor in rotating electric machines. An example of a rotating electric machine is a permanently magnetized synchronous machine having permanent magnets mounted on its rotor and a three-phase winding typically mounted on its stator. When the rotor turns, the rotating magnetic flux generated by the magnets in the rotor crosses the stator windings and generates current in the stator coils. Correspondingly, when alternating current is fed into the stator, a magnetic flux is generated in the stator coils that when rotating turns the permanently magnetized rotor.
A problem with the present permanent magnets used in electrotechnical devices is the difficult fastening and small size of the permanent magnet pieces. When manufacturing large devices, it is necessary to handle a large number of fragile and intractable permanent magnet pieces, because the rotor diameter and axle length of powerful synchronous machines can be several meters.
To facilitate the mounting of permanent magnet pieces, the current way is to glue the pieces onto a metal plate that forms the permanent magnet element to mount. The pieces need then not be mounted one by one to the mounting point, such as on the surface of the rotor. After mounting, the immobility of the permanent magnet elements is typically ensured by winding a fibreglass band around the entire rotor and finally hardening the resin in the fibreglass band in an oven. After the elements are mounted, an extra work phase is thus required to secure the mounting. Especially the resin-hardening phase is long and slows down the final completion of the device. In addition, getting the piece into an oven is hard to accomplish, if the piece, for instance the rotor, is large. The fibreglass band is longitudinally quite strong and consequently, it provides a reliable support for the magnets against centrifugal forces when the magnets are mounted on a rotating rotor, for instance. The band does not, however, provide a sideways support for the magnets. In addition, when mounting permanent magnets or permanent magnet elements, external magnetic metal pieces fasten strongly to their surfaces and hamper and slow the mounting.